1. Field of the Invention
The present invention relates to an organic electro luminescence device, and more particularly, to an organic electro luminescence device which is capable of improving the light efficiency and aperture ratio and a fabrication method thereof.
2. Description of the Related Art
One of new flat panel displays (FPD) is an organic electro luminescence device. Since the organic electro luminescence device is a self-luminous display device, it has a high contrast and wide viewing angle compared with the LCD. Also, since the organic electro luminescence device does not require a backlight assembly, it is lightweight and slim. In addition, the organic electro luminescence device can decrease power consumption.
Further, the organic electro luminescence device can be driven at a low DC voltage and has a rapid response time. Since all of the components of the organic electro luminescence device are formed of solid materials, it is endurable against external impact. It can also be used in a wide temperature range and can be manufactured at a low cost.
Specifically, the organic electro luminescence device is easily fabricated using a deposition apparatus and an encapsulation apparatus. Therefore, the fabrication method and apparatus of the organic electro luminescence device are simpler than those of an LCD or PDP.
Such a related art organic electro luminescence device is driven in a passive matrix mode that does not require separate switching elements. In the passive matrix mode, scan lines and signal lines are crossed with one another to thereby define pixels in a matrix form. In order to drive pixels, the scan lines are sequentially driven according to time. Therefore, in order to produce a desired mean brightness, the passive matrix organic electro luminescence device provides an instantaneous brightness corresponding to a product of the mean brightness and the number of lines.
In an active matrix mode, however, a thin film transistor (TFT) acting as a switching element to turn on/off pixel is disposed in each sub-pixel. A first electrode connected to the TFT is switched on/off based on the sub-pixel, and a second electrode facing the first electrode is a common electrode.
In the active matrix, since a voltage applied to the pixel is charged in a storage capacitor (CST), a voltage is applied until a next frame signal is inputted. Therefore, the organic electro luminescence device is continuously driven during a current frame period regardless of the number of scan lines.
If the organic electro luminescence device is driven in an active matrix mode, uniform brightness can be obtained even when a low current is applied. Accordingly, the active matrix organic electro luminescence device has advantages of low power consumption, high definition, and large-sized screen.
Hereinafter, a basic structure and operational characteristic of an active matrix organic electro luminescence device will be described in detail.
FIG. 1 is a circuit diagram of a related art active matrix organic electro luminescence device.
Referring to FIG. 1, a scan line 2 is formed in a first direction. A signal line 3 and a power supply line 4 are formed in a second direction crossing the first direction and spaced apart from each other by a predetermined distance. One sub pixel region is defined by the scan line 2, the signal line 3, and the power supply line 4.
A switching TFT 5 acting as an addressing element is formed at an intersection of the scan line 2 and the signal line 3. A storage capacitor CST 6 is connected to the switching TFT 5 and the power supply line 4. A drive TFT 7 acting as a current source element is connected to the storage capacitor CST and the power supply line 4. An electro luminescent diode 8 is connected to the drive TFT 7.
When a forward current is supplied to an organic luminescent material of the electro luminescent diode 8, electrons and holes moves and are recombined due to the P-N junction between an anode electrode acting as a hole providing layer and a cathode electrode acting as an electron providing layer. At this time, light is emitted.
The organic electro luminescence device is classified into a top emission type organic electro luminescence device and a bottom emission type organic electro luminescence device according to an emission direction of the electro luminescent diode 8.
FIG. 2 is a schematic sectional view of a related art bottom emission type organic electro luminescence device. In FIG. 1, only one pixel region including red, green and blue sub-pixels is illustrated for conciseness.
Referring to FIG. 2, first and second substrates 10 and 30 are arranged to face each other. Edge portions of the first and second substrates 10 and 30 are encapsulated by a seal pattern 40. A TFT T is formed on a transparent substrate 1 of the first substrate 10 in sub-pixel unit. A first electrode 12 is connected to the TFT T. An organic electro luminescent layer 14 is formed on the TFT T and the first electrode 12 and is arranged corresponding to the first electrode 12. The organic electro luminescent layer 14 contains light emission materials taking on red, green and blue colors. A second electrode 16 is formed on the organic electro luminescent layer 14.
The first and second electrodes 12 and 16 function to apply an electric field to the organic electro luminescent layer 14.
Due to the seal pattern 40, the second electrode 16 and the second substrate 30 are spaced apart from each other by a predetermined distance. Therefore, an absorbent (not shown) and a translucent tape (not shown) may be further provided in an inner surface of the second substrate 30. The absorbent absorbs moisture introduced from an exterior, and the translucent tape adheres the absorbent to the second substrate 30.
In the bottom emission type structure, when the first electrode 12 and the second electrode 16 are used as an anode and a cathode, respectively, the first electrode 12 is formed of a transparent conductive material and the second electrode 16 is formed of a metal having a low work function. In such a condition, the organic electro luminescent layer 14 includes a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and an electron transporting layer 14d, which are sequentially formed on the first electrode 12.
The emission layer 14c has red, green and blue color filters for sub-pixels.
FIG. 3 is an enlarged sectional view of one sub-pixel region in the bottom emission type organic electro luminescence device shown in FIG. 2.
Referring to FIG. 2, a semiconductor layer 62, a gate electrode 68, and source and drain electrodes 80 and 82 are sequentially formed on a transparent substrate 1, thus defining a TFT region. A power electrode 72 extending from a power line (not shown) and an electro luminescent diode E are connected to the source and drain electrodes 80 and 82.
A capacitor electrode 64 is disposed at a lower portion corresponding to the power electrode 72. A dielectric layer is interposed between the power electrode 72 and the capacitor electrode 64. A region corresponding to these layers is a storage capacitor region.
Except the electro luminescent diode E, the elements formed in the TFT region and the storage capacitor region are an array element A.
The electro luminescent diode E includes a first electrode 12, a second electrode 16, and an organic electrode luminescent layer 14 interposed between the first and second electrodes 12 and 16. The electro luminescent diode E is disposed in an emission region from which a self-luminous light is emitted.
Like this, in the related art organic electro luminescence device, the array element (A) and the electro luminescent diode (E) are stacked on the same substrate.
FIG. 4 is a flowchart illustrating a method for fabricating the related art organic electro luminescence device.
Referring to FIG. 4, an array element is formed on a first substrate (st1). The first substrate is a transparent substrate. The array element includes a scan line, a signal line perpendicular to the scan line and spaced apart from the scan line by a predetermined distance, a power supply line, a switching TFT formed at an intersection of the scan line and the signal line, and a drive TFT connected to the switching TFT.
A first electrode as a first component of the electro luminescent diode is formed and is connected to the drive TFT (st2). The first electrode is patterned in each sub pixel.
An organic electro luminescent layer as a second component of the electro luminescent diode is formed on the first electrode (st3). When the first electrode is used as the anode electrode, the organic electro luminescent layer can include a hole injection layer, a hole transporting layer, an emission layer, and an electron transporting layer, which are sequentially stacked in this order.
A second electrode as a third component of the electro luminescent diode is formed on the organic electro luminescent layer (st4). The second electrode is formed as a common electrode on the substrate.
The first substrate is encapsulated using the second substrate (st5). The second substrate protects the first substrate from external impact and prevents the organic electro luminescent layer from being damaged due to introduction of outdoor air. An absorbent may be included in an inner surface of the second substrate.
Like this, the bottom emission type organic electro luminescence device is fabricated by attaching the substrate, where the array element and the organic electro luminescent diode are formed, to the separate substrate provided for the encapsulation. In this case, the yield of the organic electro luminescence device is determined by the product of the yield of the array element and the yield of the organic electro luminescent diode. Therefore, the entire process yield is greatly restricted by the late process, that is, the process of forming the organic electro luminescent diode. For example, even though excellent array elements are formed, if foreign particles or other factors cause defects in forming the organic electro luminescent layer of a thin film of about 1000 Å thick, the corresponding organic electro luminescence device is decided as a defective grade.
Thus, the expense and material cost spent in fabricating the non-defective array element is lost, resulting in the reduction of the yield.
In addition, the bottom emission type organic electro luminescence device has high stability and high degree of freedom due to the encapsulation, but has limitation in aperture ratio. Thus, the bottom emission type organic electro luminescence device is difficult to apply to high-definition products. Meanwhile, in the case of the top emission type organic electro luminescence device, the design of the TFTs is easy and the aperture ratio is high. Thus, it is advantageous in view of lifetime of the product. However, since the cathode is disposed on the organic electro luminescent layer, the selection of material is restricted. Consequently, the transmittance is limited and the luminous efficiency is degraded.